This invention relates to the redistribution of polycarbonates, and more particularly to a redistribution method of high efficiency which is capable of producing an improved polycarbonate material.
Polycarbonates have traditionally been prepared by an interfacial method involving the reaction of a dihydroxyaromatic compound with phosgene, or a melt method in which the phosgene is replaced by a diaryl carbonate. In recent years, however, a redistribution process for use with polycarbonates has been developed. This process, described in U.S. Pat. Nos. 5,414,057 and 5,459,226, comprises heating an already formed polycarbonate in the presence of a redistribution catalyst to produce a polycarbonate with a different, generally lower, molecular weight. The method is highly useful commercially because it permits the preparation from monomeric materials of a single high molecular weight polycarbonate, which may then undergo redistribution to yield a spectrum of lower molecular weight materials which can be custom designed depending on their intended use. A similar method, applicable to polyestercarbonates, is disclosed in copending, commonly owned application Ser. No. 08/373,805.
A broad spectrum of redistribution catalysts is disclosed in U.S. Pat. No. 5,414,057. Included are tetraalkylammonium hydroxides, tetraalkylammonium acetates, phosphines such as triphenylphosphine and organometallic compounds. The use of tetraorganophosphonium carboxylates as redistribution catalysts, disclosed in U.S. Pat. No. 5,459,226, is advantageous when the presence of amine residues in the polycarbonate, formed by the decomposition of a tetraalkylammonium material, is detrimental.
Often, the use of the aforementioned catalysts affords products having more color than is desired, particularly when use of the polycarbonate as a transparent sheet material is intended. It is also frequently found that the proportion of redistribution catalyst necessary to afford a product of the desired molecular weight is higher than expected. It has further been noted that the presence of water as a constituent of the redistribution mixture is frequently necessary for the attainment of optimum results. This is particularly true when a tetraorganoammonium or tetraorganophosphonium salt such as the acetate is employed, since an initial preferred step appears to be conversion by hydrolysis to the hydroxide which increases the reaction time.
It remains of interest, therefore, to develop new and improved redistribution methods which utilize catalysts of higher efficiency than those presently known. It is also of interest to develop improved redistribution processes which result in the formation of polycarbonates having decreased color.